1. Field of the Invention
The present invention relates to a method and apparatus for estimating sensitivity change in a photomultiplier used in a radiation image reading unit constructed to read out a radiation image using the photomultiplier.
2. Description of the Related Art
Radiation image recording/reading systems that employ a storage phosphor (stimulable phosphor) are proposed. When the storage phosphor is exposed to radiation (X-ray, α-ray, β-ray, γ-ray, electron beam, ultraviolet ray, or the like), it stores a part of the radiation energy, and thereafter emits stimulated luminescence light in proportion to the radiation energy stored therein when exposed to excitation light such as visible light. In the radiation image recording/reading system, radiation image obtained by imaging a subject, such as a human body or the like, is recorded temporarily on a sheet-shaped storage phosphor sheet. Thereafter, it is scanned with excitation light, such as a light beam, to generate stimulated luminescence light, which is in turn photoelectrically read out by a reading means, such as a photomultiplier (PMT) to obtain image signals, and the radiation image of the subject is outputted on a recording material, such as a photographic sensitive material, or on a CRT display as a visible image based on the image signals.
Here, the sensitivity of the PMT (magnitude of output signal to input luminous energy) used in the radiation image recording/reading system is determined by the magnitude of the high voltage applied to the PMT. Each PMT of the same type, however, has its own characteristics which are different with each other. Thus, the voltage required for obtaining an intended sensitivity differs from PMT to PMT. Consequently, the high voltage is properly adjusted for each system to obtain an intended sensitivity at the time of factory shipment as described, for example, in Japanese Unexamined Patent Publication Nos. 3(1991)-132644 and 2001-100340.
Recently, however, it has been revealed that the sensitivity of the PMT changes as it is used. In particular, it has been demonstrated that the sensitivity of the PMT used in the radiation image recording/reading system changes significantly due to a large anode current flowing through the PMT. FIG. 10 illustrates the relationship between the cumulative amount of radiation irradiated on the storage phosphor sheet, i.e. cumulative value of anode currents flowed through the PMT and the sensitivity of the PMT. It has been demonstrated through an experiment that the sensitivity of the PMT changes to as low as approximately 1/10 as cumulative amount of radiation increases, as shown in FIG. 10.
Accordingly, even if the voltage applied to the PMT is adjusted properly at the time of factory shipment, the sensitivity of the PMT changes as it is used, so that stimulated luminescence light of small luminous energy emitted from the area of the storage phosphor sheet which has been exposed to a small amount of radiation, for example, may not be detected properly. Moreover, the sensitivity change in the PMT is not discernible simply by observing the reproduced image. That is, if the image quality of the reproduced image appears degraded, one can not tell if it is due to the sensitivity change, or the image has such quality from the beginning, which may lead to improper diagnostic image interpretation.
One method for detecting the sensitivity change in the PMT using a reference light source may be conceivable, in which the sensitivity of the PMT is detected based on the relationship between the detected signal obtained by the PMT by receiving the reference light emitted from the reference light source and the reference signal which has been predetermined according to the luminous energy of the reference light. The installation of such additional reference light source, however, requires additional costs. Moreover, it will involve an immense amount of time and effort to newly install such reference light source in the systems already installed in the hospitals and the like. Thus, installation of such additional reference light source in the existing systems is unrealistic.
In addition, the sensitivity change in the PMT is presumed to be dependent on the cumulative value of anode current flowed through the PMT, the sensitivity change in the PMT may not be estimated properly based simply on the total operation time of the PMT or number of radiation images read out by the PMT, as described for example, in Japanese Patent Publication No. 7(1995)-031373.